1. Field of the Invention
This invention relates to air diffusers and, more particularly, to air diffusers having arrangements for selectively controlling air flow based on thermal characteristics.
2. Description of Related Art
The use of air distribution units or diffusers as part of a heating, ventilating and air conditioning (HVAC) system is relatively well known. Such systems often include an air distribution system comprising a network of air ducts which supply heated or cooled air to various spacial areas in a building ventilated by the HVAC system. The air is discharged from the ducts into the ventilated areas by a series of air outlets. Typically, a diffuser is provided at or near the air outlet to control the flow of air entering the ventilated areas, and to provide an even distribution of air to a desired area without undue noise or uncomfortable drafts to disturb the occupants. The diffuser generally employs one or more vanes or deflectors to direct the discharge air flow in the desired orientation.
It is well known that the most effective manner of distributing air from diffusers during cooling is to direct the air in a horizontal flow across the ceiling of the room by employing what is known in the industry as the "Coanda effect." A stream of air discharged from a diffuser at an angle less than approximately 35 to 40 degrees with respect to the ceiling will tend to create a partial vacuum and cause the air stream to remain in contact with or hug the ceiling as a result of the Coanda effect. Cool air, therefore, can be distributed over a substantial area of the ceiling by employing the Coanda effect before the cool air begins to "sink down" into the room to cool the entire volume of the room.
In contrast, the best efficiency in connection with the distribution of hot air from ceiling mounted diffusers is for the air to be discharged in a generally vertically oriented stream. The vertical flow component overcomes the natural buoyancy of the heated discharge air to serve the lower strata of the room with heated air and to thereby achieve a proper mixing of the heated air with the room air. Because the discharged air flows through the occupied space before mixing is complete, drafts may be directed at the occupants. However, most people do not perceive warm drafts as uncomfortable.
In many HVAC applications, the same duct outlet may selectively discharge either heated or cooled air during different times of the day. For instance, thermostats are typically set back to 65.degree. F. or lower during the middle of the night. In the morning, most of the duct outlets will be discharging heated air to bring the air in the building up to its desired daytime temperature setting. As the day progresses, interior heat loads from people, lighting and computers may require that cooled air be directed to the interior of the building. Also, a southern exposure of the building will likely experience a solar warming load which must be overcome with cooled air directed to that part of the building. Particularly in the Spring and Fall, all areas of the building may require heated air in the morning and cooled air during the middle of the day.
While it is desireable to change the orientation of the air exiting the duct outlet, it is inconvenient to manually adjust a diffuser several times during the course of the day. Further, some buildings, such as banks and shopping malls, have their duct outlets located in high ceilings. Even infrequent adjustments of a manual diffuser is inconvenient in such locations. To address these problems, diffuser or air distribution units have been previously developed which automatically adjust the air distribution from a diffuser. Typically these systems employ some form of control system to sense the temperature of the discharge air and to control motorized adjustable vanes in the diffusers to adjust the air flow.
Several diffusers or air distribution units have been developed which incorporate the concept of a non-motorized, thermally responsive element to adjust the air distribution. For instance, in Kline et al., U.S. Pat. No. 4,515,069 issued May 7, 1985, a thermally responsive actuator rotates a blade assembly so that when warm air is flowing through the duct, a hot blade directs air primarily downward. When cold air is flowing through the duct, a separate cold blade directs the air primarily across the ceiling. The blades in Kline et al. comprise generally vertically oriented, opposite sides of a diamond-shaped box. By changing the orientation of the box in a duct, either the hot or cold blade may be presented to the air flow. A path formed between the selected blade and an opposing wall of the duct directs the air flow in the desired direction. The cold blade is angled more sharply off the vertical axis and interacts with a ramp wall formed in the duct to direct the air flow across the ceiling, while the hot blade is angled less sharply off the vertical axis to direct the air flow generally vertically downward.
The angle of the box in Kline et al. is controlled by a thermally responsive actuator or "pill" located in the flow path to sense the temperature of the air flow discharging the diffuser. The pill has a cavity filled with wax which expands upon melting at a predetermined temperature, thereby causing a piston to move outwardly from the cavity. At cooler temperatures, the wax solidifies and contracts. The piston is connected to the box by a linkage, so that movement of the piston into and out of the pill provides the motive force to operate the diffuser without the aid of additional motors and the like.
Two somewhat related patents, Herb, U.S. Pat. No. 4,535,932 issued Aug. 20, 1985, and Bryans, U.S. Pat. No. 4,625,629 issued Dec. 2, 1986, each employ a pill to operate a diffuser which selects between two discharge outlets depending upon the temperature of the discharge air. One discharge outlet directs the flow against a wall, and the other discharge outlet directs the flow into the interior of a room. The pill directs a baffle to selectively block one of the discharge outlets.
A separate concept for controlling the vertical and horizontal flow components of the discharge air is disclosed by Kennedy, U.S. Reissue Pat. No. 25,216 reissued Aug. 7, 1962. Kennedy controls the vertical and horizontal flow components of the discharge stream by selectively obstructing the central flow path of the air exiting the diffuser. With the central flow path blocked, the discharge flow is substantially horizontal. With the central flow path unobstructed, the discharge flow is substantially vertical.
The Kennedy diffuser comprises a perforated deflector of smaller area than, and in a substantially parallel orientation to, a perforated duct outlet face plate. The position of the deflector is controlled by a bellows which expands and contracts in response to the temperature of the air passing through the duct. In the warm air position, the perforations in the deflector and face plate are in register and allow air to pass through the center of the face plate. In the cold air position, the perforations and the deflector and face plate are out of register, preventing air from passing through the center of the face plate and forcing the air to exit the face plate with a more horizontal orientation. By blocking the air passageways, the deflector of Kennedy restricts the volume of air passing through the face plate when it is in the cold position.
Due to the large number of diffuser outlets in any given building, it is desirable that each be relatively simple and inexpensive. If adjustable diffuser outlets are used, it is desireable that each be quick and easy to adjust to accommodate variable air discharge temperatures. Automatically adjustable diffusers effectively address the latter requirement. However, many of the prior art automatic temperature controlled diffusers are unnecessarily complicated, employing sophisticated control systems and motorized diffuser elements to automatically select the appropriate flow pattern for a given temperature of discharge air.
An additional limitation of many prior art devices is the reduction of the area of the duct outlet when adjusted to discharge cooled air, thus reducing the volume of air discharging the diffuser. This has several disadvantages. For example, cooled air discharge temperatures are typically much closer to the desired building temperature than heated air discharge temperatures. Obtaining a given amount of cooling in a building space therefore requires proportionally more discharge air volume than an energy equivalent amount of heating in the same space. Reducing the area of the diffuser when discharging cooled air reduces the volume of discharge air precisely when an increased volume is desired.
Another instance where it is undesirable to reduce the volume of discharge air through individual diffusers is when using a variable air volume (VAV) control system. VAV systems employ dampers in the ductwork to direct a varying volume of air to a particular building space so as to achieve a desired air temperature in that space. An air diffuser which further restricts the volume of air entering the space makes control of the VAV system more difficult. For example, the air flow across a restricted duct outlet may create excessive noise.